1. Field of the Invention
The present invention relates to a color television display apparatus having improved convergence of electron beams, by which leakage flux caused by vertical deflection coils unfavorably affecting the convergence of electron beams can be removed.
2. Description of the Prior Art
In a color television display apparatus, it is generally recognized that a trailing phenomenon in movement of a picture near the end portions of a screen, namely, so-called flare occurs. Such flare can be avoided by using a magnetically-permeable material for a focusing electrode of an electron gun instead of a non-magnetic material as conventionally utilized. However, such use of a magnetically-permeable material causes another problem that convergence of a display apparatus deteriorates. This is because the leakage flux caused by the vertical deflection coils provided for effecting vertical deflection of the electron beams is formed in a pincushion shape in the focusing electrode of the electron gun, exerting unfavorable influence on the focusing of the electron beams.
Accordingly, means for compensating for the deterioration of convergence namely, misconvergence, in case of using such a magnetically-permeable focusing electrode was proposed in the specification of U.S. Pat. No. 4,335,366 issued June 15, 1982 to Roger C. Alig et al and assigned to RCA Corporation, New York, N.Y., U.S.A. A magnetic member for compensation for misconvergence as proposed in the above stated patent comprises a first portion for collecting leakage flux of the vertical deflection coils and a second portion disposed approximately perpendicular to a neck region of a cathode ray tube, for supplying the collected vertical leakage flux to the vicinity of the exit end of the electron beams.
However, the above stated first portion covers approximately half of the exterior of the vertical deflection coils, which involves a disadvantage that such a magnetic member as described above needs be of a large size. In addition, in such a magnetic member located externally to the vertical deflection coils, there is another disadvantage that a satisfactory effect of compensation for misconvergence cannot be obtained.
Furthermore, in case of a display apparatus being of a type in which a magnetic piece for channelling a vertical leakage field to the front end of a deflection yoke, namely a so-called cross arm, as proposed for example in U.S. Pat. No. 4,257,023 issued Mar. 17, 1981 to Naoyoshi Kamijo et al and assigned to Tokyo Shibaura Denki Kabushiki Kaisha, Kawasaki, Japan is provided for the purpose of correcting the pincushion distortion of a raster, if a large-sized magnetic member as already proposed above is attached, the above stated cross arm and the attached magnetic member would magnetically interact with each other, resulting in hindrance to the achievement of the desired effects.
As another proposal for compensation of misconvergence, an invention described in Japanese Utility Model Publication No. 18210/1983 may be preferred. This invention comprises a core about which vertical deflection coils are wound and a pair of magnetic pieces coupled to the core and protruding backward from the core. With such pair of magnetic pieces protruding backward, the center of the magnetic field of the vertical deflection coils can in appearance deviate backward in the cathode ray tube, whereby the deflection sensitivity can be improved. In other words, the above mentioned invention intends to exclude the unfavorable influences due to the leakage field by deviation from the center of the magnetic field without changing the absolute quantity of effective magnetic flux produced by the vertical deflection coils.
On the other hand, in the invention of the present application or the above stated U.S. Pat. No. 4,335,366, leakage flux is received by a magnetic member or the like and the received flux is transmitted to a predetermined place as an effective magnetic flux and, therefore, the objects and the structure of the present invention or the above stated U.S. patent are different from those of the above stated Japanese Utility Model Publication No. 18210/1983.